Synchronous machine with power and voltage control

ABSTRACT

A synchronous machine for power and/or voltage control comprises a stator with a stator winding and a rotor with a field winding. The stator winding comprises a high-voltage cable with solid insulation. A rotor has a thermally based rotor current limit intersecting with a thermally based stator current limit in a capability graph at a power factor considerably below the rated power factor or has the thermally based rotor current limit above the thermally based stator current limit. In the capability graph. Means are provided for limiting the currents in order to avoid thermal damage. In a method for power and/or voltage control of such a synchronous machine, the machine operates with the stator current exceeding the thermally based stator current limit for a certain time period less than the maxim an permissible time limit, whereafter the overload is reduced by reduction of either the active power or the field current or a combination of both.

The present invention relates to a method for power and/or voltagecontrol in a synchronous machine, and a synchronous machine for powerand/or voltage control.

In the following “synchronous machine” shall be taken to meansynchronous generator. Synchronous generators are used in electric powernetworks in the first place to supply active and reactive power in the“hour scale”. Active power can also be controlled in the “second-minutescale” (frequency control), as well as reactive power (voltage control).Synchronous machines also provide suitable contributions in the“millisecond scale” to the fault currents, so that error states in thenetwork can be quickly resolved in selective manner.

Synchronous machines are important production sources of reactive powerin power systems. When the reactive power requirement increases in thesystem, this tends to lower the terminal voltage on the synchronousmachine. To keep the voltage constant, the field current is normallyincreased by means of the voltage regulator of the synchronous machine.The synchronous machine will thus produce the reactive power required toachieve reactive power balance at the desired terminal voltage.

The above mentioned process applies as long as the power productioncorresponds to one point in the permissible area in the capability graphof the synchronous machine, i.e. the graph of limits as regards reactiveand active power, see FIG. 1 showing the relationship at overexcitedoperation. At overexcited operation, i.e. when the synchronous machineis producing reactive power, the permissible operating area is limitedby thermally based rotor and stator current limits. The synchronousmachines of today are normally dimensioned so that rotor and statorcurrent limits intersect each other at a point at rated power factor A,see FIG. 1. The rated power factor for synchronous generators istypically 0.8-0.95. At overexcited operation, where the power factor isgreater than the rated power factor, the limit for the capability graphof the synchronous machine consists of the stator current limit and, atoverexcited operation, where the power factor is less than the ratedpower factor, the limit consists of the rotor current limit.

In conventional technology, if the stator or rotor current limits areexceeded current limiters, if such are installed and used, come intooperation. These limiters reduce the currents by lowering theexcitation. Since it takes a certain time before damaging temperaturesare obtained, intervention of the current limiters of the stator orrotor is delayed several seconds before the current is lowered. Thedelay typically depends on the size of the current but it is usuallyless than one minute, see e.g. VERIFICATION OF LIMITER PERFORMANCE INMODERN EXCITATION CONTROL SYSTEMS in IEEE Transaction on EnergyConversion, Vol. 10, No. 3, September 1995. The current reduction isachieved by a decrease in the field current which results in a decreasein the terminal voltage and reactive power production of the generator.The consequences for the part of the system in the vicinity of themachine are that the local reactive power production decreases and thatit is more difficult to import power from adjacent parts of the system,when the voltage drops.

If the transmission network is unable to transmit the power required atprevailing voltages there is a risk of the power system being subjectedto voltage collapse. To avoid this it is advantageous for the power tobe produced locally, close to the load. If this is not possible, and thepower must be transmitted from other parts of the system, it is, asknown, advantageous if this can be done at as high a voltage level aspossible. When the voltage drops, the reactive power production (shuntcapacitances) of the transmission lines decrease. Transformertap-changers act in order to keep the voltages to the loads constant,and thus the power of the loads constant. If the power consumption ofthe loads is constant and the transmission voltage is lower thannormally, the currents in the transmission lines will be higher and thereactive power consumption of the transmission lines will be greater(series inductances), see Cigré brochure 101, October 1995.

In many power systems, if current limiters come into operation forcertain synchronous machines as described above, the reactive powerproduction is limited and this may lead to a voltage collapse of thesystem.

In normal operation of the power system, with an essentially intactnetwork, these situations are normally avoided by the installation ofadditional reactive power production resources, e.g. mechanicallyswitched shunt capacitors and/or thyristor controlled static varcompenstors (SVC), if necessary. However, as a widespread voltagecollapse usually has severe consequences for the society, also abnormaloperating conditions needs to be considered. If the network is weakened,due to e.g. faults or maintenance on important elements of the network,the installed reactive power producing resources may no longer besufficient, resulting in the above described situation which may lead tovoltage collapse. The cost of installing additional controllablereactive power producing resources, e.g. SVC devices, such that alsothese abnormal operating conditions can be handled is considerable.There is consequently a need for inexpensive controllable reactive powerproduction reserves. These reserve resources should be capable ofdelivering reactive power such that voltage can be maintained atprescribed levels for at least 10 to 20 minutes giving the systemoperators a chance to take preventive actions, such as e.g. starting gasturbines or shedding load.

In power systems known today, or in power plants, the energy conversionusually occurs in two stages, using a step-up transformer. The rotatingsynchronous machine and the transformer, each have a magnetic circuit.It is known that manufacturers of such equipment are cautious andconservative in their recommendations for the set values in the limitdevices, see Cigré brochure 101, October 1995, section 4.5.4., page 60.Coordination is required and a certain risk of conflict thus exists indimensioning and protecting generators and transformers. The step-uptransformer has no air gap and is therefore sensitive to saturation as aresult of high voltage or geomagnetic currents. The transformer alsoconsumes part of the reactive power of the generator, both at normal andabnormal operation. The majority of the active losses appear in theconductors of the armature circuit and the step-up transformer, whilethe core losses are relatively small in both devices. One complicationhere is that the losses are normally developed at medium and highvoltage and are therefore more difficult to cool away than if they hadbeen developed at earth potential.

The object of the present invention is to achieve a synchronous machinefor power and/or voltage control and a method for power and/or voltagecontrol in order to avoid voltage collapse in power systems.

According to the invention, thus, the synchronous machine is designed sothat the thermally based rotor current limit is raised with respect tothe thermally based stator current limit such that either theintersection with the thermally based stator current limit in thecapability graph is at a power factor value considerably below powerfactor value, or the rotor current limit is raised above the statorcurrent limit such that the two limits do not intersect. If the rotorand stator current limits intersect at the power factor zero in thecapability graph as shown in FIG. 2, or if the rotor current limit israised above the stator current limit, the stator current limit will belimiting for all overexcited operation.

In the following “cable” shall refer to high-voltage, insulated electricconductors comprising a core having a number of strand parts ofconducting material such as copper, for instance, an innersemiconducting layer surrounding the core, a high-voltage insulatinglayer surrounding the inner semiconducting layer, and an outersemi-conducting layer surrounding the insulating layer. A synchronousmachine with a stator winding which comprises this type of cable can bedesigned for direct connection to the power network at higher voltagesthan with conventional machines, thus eliminating the need for a step-uptransformer. In the case of reactive power production it is advantageousto use a machine designed for direct connection to transmission level,since the reactive power consumed in the step-up transformer in theconventional plant instead can be delivered to the power network with amachine according to the invention.

The advantages of the invention are particularly noticeable in a machinewound with a cable of the type described above, particularly a cablehaving a diameter within the interval 20-200 mm and a conducting areawithin the interval 80-3000 mm². Such applications of the invention thusconstitute preferred embodiments thereof.

Raising the rotor current limit has a number of advantages for asynchronous machine. It enables direct measurement of limiting statortemperatures, for instance. This is considerably more difficult if thelimiting temperatures are located in the rotor since it is difficult tomeasure, or in any other way communicate with a rotating object.Furthermore, reducing active power enables more reactive power to beproduced. This is also possible with conventional rotor dimensioning butmore MVAr per reduced MW results in this case, as can be seen in thecurves in FIGS. 1 and 2.

A number of other advantages are also gained by raising the rotorcurrent limit, specific to this type of machine. The time constants forheating (and cooling) the stator are large in comparison with aconventional machine. This means that the machine, with conventionalstator current limiters, can be run overloaded or longer than aconventional machine without damaging temperatures being reached.Simulations indicate that the stator safely can be overloaded 80% for 15minutes in some cases. This extended time period can be utilized to takeaction either to reduce the system's need for reactive power, or toincrease the production of reactive power. It is also easier toimplement forced cooling of the stator of the machine. A machine of thistype has a degree of efficiency comparable with that of a conventionalmachine, i.e. the stator losses are approximately equivalent. While aconventional machine has primarily conductor losses, this type ofmachine has less conductor losses and more core losses. Since the corelosses are developed at earth potential they are easier to cool away. Acooling machine can be used, for instance, for forced cooling insituations with high core temperatures.

With conventional current limiters the time period contributed by thetime constant for heating, can be utilized to reduce the active powerand thus enable increased and/or prolonged production of reactive power.The need for reducing the field is thus less and, in the best case, iseliminated.

With direct temperature measurement or temperature estimation (or acombination thereof) we can pass from using the term “stator currentlimit” to talking about stator temperature limit(s). Since it is thestator temperature (in critical points), and not the stator current,that is limiting, this offers a number of advantages. The generaltendency to set the limiter conservatively can thus be lessened since itis the primary quantity that is known and not a derivative. With aconventional current limiter no consideration can be taken to thetemperature of the machine when the current limit is exceeded, i.e. noconsideration can be taken to the fact, for instance, that the machinewas started shortly before the current limit was exceeded, or that theload was low shortly before. This can be avoided by using statortemperature limit(s) instead. Cooling of the machine is dimensioned sothat the stator in continuous rated operation does not exceed a certaintemperature—let us call this the rated temperature. This temperature isconsciously set conservatively, i.e. the stator (insulation) canwithstand higher temperatures for long periods of time. If thetemperature in the critical points is known the machine can be run aboverated operation for relatively long periods.

Dimensioning the rotor with salient poles (hydroelectric generators) insynchronous machines according to the invention is facilitated by thefact that the inner diameter of the stator can be made larger than inconventional machines since the stator winding is composed of cable inwhich the insulation takes up more space. It is thus possible to designthe stator for this type of synchronous machine in accordance withconventional dimensioning procedures and change only the design of therotor so that the rotor current limit is raised in the desired manner.

For a synchronous machine incorporating an air-cooled rotor with salientpoles, this can be done, for instance, by utilizing the extra space towind extra turns of the field winding in order to increase the magneticpole voltage. A certain number of turns in the field winding thenconsist of cooling turns, thus increasing the cooled surface of thefield winding. If the extra turns are provided with the same proportionof cooling turns, as the other turns the temperature increase in thefield winding can be kept at the same level as in a conventionaldimensioning procedure, despite the magnetic pole voltage being raised.

For a synchronous machine with cylindrical rotor (turbo-rotor) the rotorcurrent limit can be increased by making the machine longer, forinstance.

The invention will now be explained in more detail in the following withreference to the accompanying drawings in which

FIGS. 1 and 2 show capability graphs for overexcited synchronousmachines with conventional dimensioning and in accordance with theinvention, respectively,

FIG. 3 shows a cross section through the cable used for the statorwinding in the synchronous machine according to the invention,

FIG. 3A is a schematic illustration of a machine in accordance to thepresent invention illustrating a stator, rotor, a winding and atemperature sensor in a stator slot,

FIGS. 4 and 5 show two embodiments of a temperature estimator in thesynchronous machine according to the invention.

FIG. 6 shows an example of a temperature-monitoring circuit that emitsan output signal for further control, and

FIGS. 7-9 show various circuits for control of the synchronous machineaccording to the invention.

FIG. 3 shows a cross section through a cable used in the presentinvention. The cable is composed of a conductor consisting of a numberof strand parts 2 made of copper, for instance, and having circularcross section. This conductor is arranged in the middle of the cable 1and around the conductor is a first semiconducting layer 3. Around thefirst semiconducting layer 3 is an insulating layer, e.g.XLPE-insulation, and around the insulating layer is a secondsemiconducting layer that is normally earthed.

In the machine according to the invention the windings are thuspreferably cables of a type having solid, extruded insulation, such asthose used nowadays for power distribution, e.g. XLPE-cables or cableswith EPR-insulation. Such cables are flexible, which is an importantproperty in this context since the technology for the device accordingto the invention is based primarily on winding systems in which thewinding is formed from cable which is bent during assembly. Theflexibility of a XLPE-cable normally corresponds to a radius ofcurvature of approximately 20 cm for a cable 30 mm in diameter, and aradius of curvature of approximately 65 cm for a cable 80 mm indiameter. In the present application the term “flexible” is used toindicate that the winding is flexible down to a radius of curvature inthe order of four times the cable diameter, preferably eight to twelvetimes the cable diameter.

Windings in the present invention are constructed to retain theirproperties even when they are bent and when they are subjected tothermal stress during operation. It is vital that the layers retaintheir adhesion to each other in this context. The material properties ofthe layers are decisive here, particularly their elasticity and relativecoefficients of thermal expansion. In a XLPE-cable, for instance, theinsulating layer consists of cross-linked, low-density polyethylene, andthe semiconducting layers consist of polyethylene with soot and metalparticles mixed in. Changes in volume as a result of temperaturefluctuations are completely absorbed as changes in radius in the cableand, thanks to the comparatively slight difference between thecoefficients of thermal expansion in the layers in relation to theelasticity of these materials, radial expansion can take place withoutthe adhesion between the layers being lost.

The material combinations stated above should be considered only asexamples. Other combinations fulfilling the conditions specified andalso the condition of being semiconducting, i.e. having resistivitywithin the range of 10⁻¹-10⁶ ohm-cm, e.g. 1-500 ohm-cm, or 10-200ohm-cm, naturally also fall within the scope of the invention.

The insulating layer may consist, for example, of a solid thermoplasticmaterial such as low-density polyethylene (LDPE), high-densitypolyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethylpentene (PMP), cross-linked materials such as cross-linked polyethylene(XLPE), or rubber such as ethylene propylene rubber (EPR) or siliconrubber.

The inner and outer semiconducting layers may be of the same basicmaterial but with particles of conducting material such as soot or metalpowder mixed in.

The mechanical properties of these materials, particularly theircoefficients of thermal expansion, are affected relatively little bywhether soot or metal powder is mixed in or not—at least in theproportions required to achieve the conductivity necessary according tothe invention. The insulating layer and the semiconducting layers thushave substantially the same coefficients of thermal expansion.

Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graftpolyethylene, ethylene-butyl-acrylate-copolymers andethylene-ethyl-acrylate copolymers may also constitute suitable polymersfor the semiconducting layers.

Even when different types of material are used as base in the variouslayers, it is desirable for their coefficients of thermal expansion tobe substantially the same. This is the case with combination of thematerials listed above.

The materials listed above have relatively good elasticity, with anE-modulus of E<500 MPa, preferably <200 MPa. The elasticity issufficient for any minor differences between the coefficients of thermalexpansion for the materials in the layers to be absorbed in the radialdirection of the elasticity so that no cracks appear, or any otherdamage, and so that the layers are not released from each other. Thematerial in the layers is elastic, and the adhesion between the layersis at least of the same magnitude as the weakest of the materials.

The conductivity of the two semiconducting layers is sufficient tosubstantially equalize the potential along each layer. The conductivityof the outer semiconducting layer is sufficiently great to enclose theelectrical field in the cable, but sufficiently small not to give riseto significant losses due to currents induced in the longitudinaldirection of the layer.

Thus, each of the two semiconducting layers essentially constitutes oneequipotential surface and the winding, with these layers, willsubstantially enclose the electrical field within it.

There is, of course, nothing to prevent one or more additionalsemiconducting layers being arranged in the insulating layer.

As mentioned above, the stator current limit is thermally restricted inthe present invention. It is the insulation 4 that sets the limit in thefirst place. If a cable with XLPE-insulation is used, the temperature ofthe layer between the conductor and the insulation should not exceed 90°C., which is the maximum temperature at rated operation and normallocation in earth, for instance, i.e. the insulation can withstand thistemperature for several hours and it may be briefly somewhat exceeded.The temperature of the surface layer between the insulation and the ironin the stator should not exceed a temperature limit of typically 55° C.,i.e. the temperature difference over the insulation will be at least 35°C.

A synchronous machine according to the invention is schematicallyillustrated in FIG. 3A wherein the machine includes a rotor R, stator S,a winding W and one or more temperature determining members or sensor T.The machine may be dimensioned for a temperature of 70-80° C. in theconductor and a core temperature of 40-50° C. at rated operation. Thesetemperatures are extremely dependent on the temperature of the coolant.A cooling machine may be used to lower this temperature although innormal operation this has a negative effect on the degree of efficiency.On the other hand, connection of such a machine may be justified in anemergency situation, although it must be taken into consideration thatit may take relative.

In order to make maximum use of the thermal inertia in the stator in asynchronous machine according to the invention it is desirable for thesurrounding conductor and iron temperatures to be determined in the partof the insulation most critical from the heating aspect. This can beachieved by direct measurement using measuring devices, or with atemperature estimator of the type shown in FIG. 4. It is also possibleto combine temperature measurement and temperature estimation accordingto FIG. 5.

In FIG. 4 losses in conductors caused by the stator current, and thusdependent on the machine's loading, are represented by a current sourcePLE, and the core losses caused by the flux (voltage), which are more orless constant irrespective of the load, by a current source P_(FE). Thetemperature of the coolant is represented by the voltage source T_(KY).R_(R+S) represents thermal resistance for cooling tubes and siliconfilling, R_(ISO) thermal resistance for the insulation and C_(LE),C_(ISO) and C_(FE), the thermal capacitance for conductor, insulationand core. T_(LE) in point 54 represents the temperature in the conductorand T_(ISO) in point 52 the mean temperature of the insulation. Themodel shown in FIG. 4 can be calibrated by comparison of T_(FE) withdirectly measured iron temperature. The temperature T_(LE) is relativelydifficult and expensive to measure directly since the conductor isnormally at high potential.

The model shown in FIG. 4 can also be refined by dividing the thermalresistance between conductor and iron into several resistances connectedin series, which would correspond to different radii of the insulation.By placing a capacitance from a point between each consecutiveresistance and a reference temperature, 0° C., any temperaturedependence of the thermal capacitance of the insulation can be modelledmore precisely. Since a temperature gradient exists in the insulation,such a division would result in a somewhat improved result.

In FIG. 4 the temperatures T_(LE), T_(ISO) and T_(FE) are considered asstates whereas T_(KY), P_(LE) and P_(FE) are considered as inputsignals. The initial state values are needed to start the temperatureestimator and the estimator is normally started simultaneously with themachine, i.e. from cold machine.

The number of nodes can of course be increased, but the embodimentsdescribed in connection with FIG. 4 and below in connection with FIG. 5are to be preferred.

FIG. 5 shows a modification of the temperature estimator in FIG. 4, inwhich the iron temperature T_(FE) is measured directly. The irontemperature will then be represented by a voltage source T_(FE) in thethus simplified diagram, and serves as input signal, together withP_(LE). The temperatures T_(ISO) and T_(LE) constitute states and areobtained in the points 52 and 54 in the same way as in FIG. 4.

The copper losses are dependent on the stator current and thus on howheavily loaded the machine is. The iron losses are dependent on theflux, which is more or less constant at terminal voltage, depending onthe load. The time constant for the temperature increase and cooling ofthe core circuit is, on the other hand, extremely large in this type ofmachine and the machine therefore has greater overload capacity if ithas just been started.

Both the iron losses and the copper losses will decrease if the field isreduced.

An advantage of the synchronous machine according to the invention incomparison with a conventional machine is that the electric losses aremore associated with the flux in the core than with currents in theconductors in the armature circuit. The core losses are developed atearth potential, which facilitates normal cooling and even forcedcooling with cooling machines. The conductors of the armature circuithave relatively low current density and the losses on the high-voltagepotential are relatively small.

The time constant for heating—and thus cooling—the core circuit isextremely large. Calculations show that the adiabatic temperatureincrease occurs in the order of hundredths of °K/s. The temperatureincrease in the armature circuit is also somewhat elevated as a resultof the great thermal resistance in the solid insulation of the windingcable. At the current densities in question the adiabatic temperatureincreases by {fraction (1/30)} to {fraction (1/100)}°K/s, whileconventional machines have an adiabatic temperature increase in theorder of {fraction (1/10)} °K/s. Both the temperature in the conductorT_(LE), and in the core T_(FE) must be monitored and FIG. 6 illustratesan example of a monitoring circuit that emits an output signal forfurther control. This circuit thus comprises a temperature estimator 2according to FIG. 4, to which the input magnitudes I (stator current), U(terminal voltage) and T_(KY) are supplied. The output signals T_(LE),and T_(FE) are obtained from the estimator 2, these being compared at 4and 6, respectively, with pre-set limit values T_(L, LE), and T_(L, FE),as mentioned above, and the result of the comparison is supplied to agate 8 (Lowest Value Gate). This gate emits a control signal at itsoutput constituting the temperature difference between temperature andtemperature limit which is greatest in absolute terms. If T_(FE) ismeasured directly, only T_(LE), need be determined from I and T_(FE)with the aid of the temperature estimator. If both T_(FE) and T_(LE),are measured directly, no temperature estimator is required and themeasured temperatures are instead compared directly with the limitvalues.

FIG. 7 shows in block diagram form an example of a 2: control circuitfor reducing the active power if the stator current exceeds a maximumpermissible limit value.

A synchronous generator G is connected to a power network via a breaker10. The generator G is excited via a thyristor-rectifier 12. The voltageU is supplied via a voltage transformer PT_(S) to a measured valueconverter 14, a unit I_(L)“Prod” for determining of the actual statorcurrent limit I_(L), and to a unit ΔP“Prod” for generating a signal “ΔPorder” for reducing the active power if the stator current exceeds thestator current limit. In the same way, the current I˜ is supplied via acurrent transformer CT_(S) to the units I_(L) “Prod” and “ΔP Prod”. Inthe unit I_(L) “Prod” the direction of the reactive power, voltage dropand initial time delay allowed for reducing the field are taken intoconsideration when determining the stator current limit. The statorcurrent limit is based on the stator temperature at rated operation(T_(LE), =70-80° C. and T_(FE)=40-50° C. with XLPE-insulation). The rateof reduction and maximum range for the reduction of the active power isalso determined in the unit ΔP“Prod”, as well as a function, if any, forreturning to the active power production the synchronous machine hadbefore the stator current limit was exceeded, if the reactive powerrequirement of the system again decreases.

The maximum reactive power the synchronous machine in the embodimentdescribed can produce in steady state operation is equivalent to 100% ofrated power and is obtained when the active power has been reduced tozero. However, there is cause to introduce a lower limit greater thanzero for reducing of active power, since further reduction of activepower gives little in return of increased ability to produce reactivepower, see FIG. 2. If more reactive power is required in steady stateoperation, this must be meet by a reduction of the field after anappropriate time delay.

The output signal U from the network converter 14 is compared at 16 witha predetermined reference value U_(REF) and the result of the comparisonis supplied to an amplifier and signal-processing unit 18 before beingsupplied to a gate 20.

At 22 the stator current I is compared with the stator current limitI_(L) generated in the unit I_(L)“Prod”, and the result of thecomparison is supplied to an amplifier and signal-processing unit 24 anda subsequent block 26 with non-linear characteristic. The non-linearcharacteristic is such that a large output signal is obtained forpositive input signals and an output signal proportional to the inputsignal for negative input signals. The output signal from the block 26is also supplied to the gate 20 which is a Lowest Value Gate, i.e. thesignal that is lowest is obtained as output signal.

The output signal from the gate 20 is supplied to a signal-processingunit 28 with integrating action which is in turn connected to a triggercircuit 30 for the rectifier 12 of the excitation machine.

The control circuit in FIG. 7 comprises essentially three main parts: anautomatic voltage regulator, a stator current limiter and a system forreducing the active effect in order to increase the ability of thesynchronous machine to meet the system's demand for reactive effect atthe desired voltage level.

Reduction of the field current can be achieved in several ways accordingto the invention. A traditional limiter may thus be used that operateson the principle that if the stator current exceeds the stator currentlimit during a maximum permissible period, the field current is lowerede.g. ramped in accordance with a selected ramp function, (not shown)until the stator current becomes equal to the stator current limit.

The actual control may be effected in various ways. In this case theinitial time delay must be at least long enough to ensure that brieflarge currents arising out of error conditions in the system do notcause reduction of the field because the current limit has beenexceeded. Various methods of time delay are possible, e.g. a constantdelay time irrespective of by how much the current exceeds the limit, orinverse time characteristic, i.e. the more the current exceeds thelimit, the shorter the time delay. If the stator current limit has beenexceeded, a period of time must be allowed for cooling. The type ofsynchronous machine under consideration has large time constants withregard to heating and cooling of the stator and the time delay cantherefore be large in comparison with in the case of a conventionalmachine. This is because time is allowed either to reduce the system'sdemand for reactive power or increase the machine's ability to producereactive power.

The dimensioning of the machine, together with reduction of active powerincreases the machine's ability to produce reactive power.

According to the invention reduction of the field current is alsopossible starting from the temperature at the most critical points. Thetemperature of the conductor in the stator and the core temperature inthe stator at the most critical points can be determined either throughdirect measurement, which may be difficult in the case of conductortemperature, or with the aid of a temperature estimator with copperlosses (stator current), iron losses (voltage) and coolant temperatureas input signals, as discussed above. Two modes are thus possible forcontrol, namely:

1) if the temperatures are below their maximum permissible temperaturelimits the field current is controlled so that the terminal voltagebecomes equal to the desired operating voltage, and

2) if the terminal voltage is less than the desired operating voltage,the field current is controlled so that the conductor temperature orcore temperature becomes equal to the maximum permissible temperaturelimit and the other temperature is below its limit.

The machine of the invention is capable of operating at overload withthe stator current exceeding the thermally based stator current limit byat least 30 percent for at least three minutes.

The machine of the invention is capable of operating at overload withthe stator current exceeding the thermally based stator current limit byat least 30 percent for at least three minutes without thermal damagewherein the machine has achieved rated temperature prior to the overloadcondition. In another embodiment, the machine is capable of operating asabove with the stator current exceeding the thermally based statorcurrent limit by at least 30 percent for at least five minutes withoutthermal damage. In yet another embodiment, the machine is operable asabove with the stator current exceeding the thermally based statorcurrent limit by at least 50 percent for at least fifteen minuteswithout thermal damage. In yet another embodiment, the machine isoperable as above with the stator current exceeding the thermally basedstator current limit by at least 80 percent for at least fifteen minuteswithout thermal damage.

The transition point where the stator temperature is equal to themaximum permissible stator temperature and the terminal voltage is equalto the desired operation voltage can be realized with a Lowest ValueGate, as described in connection with the figure.

Mode 1 above corresponds to normal voltage control, whereas mode 2protects the machine against high temperatures since terminal voltageand stator temperature decrease when the field current decreases.

FIG. 8 shows a control circuit for achieving control of theabove-mentioned type.

Besides the current I˜ and the voltage U˜, the unit ΔT“Prod” is alsosupplied with the temperature T_(KY) of the coolant. The output signalfrom the unit ΔT“Prod” is supplied to an amplifier and signal-processingunit 40 and the block 26 with non-linear characteristic, as describedearlier, for supply to the gate 20 together with the processed andamplified output signal from comparison of the voltage U with desiredoperation voltage Uref. Depending on the output signal from the gate 20,control of the machine is then carried out in a manner corresponding tothat described in the embodiment according to FIG. 7.

If the limiting temperature (T_(LE) or T_(FE)) approaches its maximumtemperature limit (e.g. T_(L,LE)=90° C. and T_(L,FE)=55° C. withXLPE-insulation) with a time derivative greater than zero, the abovecontrol may result in an “over-swing” in the temperature. If thisover-temperature is brief, and providing it is moderate, it does notconstitute a serious risk to the insulation. However, it may result in atemporary voltage drop that may upset the stability of the power system,as a result of the control circuit attempting to counteract theover-temperature by reducing the field.

To avoid this, the control circuit may be supplemented with atemperature predicting circuit, e.g. based on the time derivative of thetemperature, so that even before maximum temperature is reached, thevoltage is permitted to gently start falling. The “over-swing” intemperature will then be slight, or altogether eliminated.

The voltage will thus commence falling earlier, but not so quickly.

A comparison between a traditional current limiter according to FIG. 7and a stator temperature limiter according to FIG. 8 shows the latter tohave the advantage of allowing overload over a long period of time, inthe order of hours, whereas the traditional current limiter only permitsoverload for a short period of time, in the order of seconds-minutes.

If the machine is equipped with stator temperature limiters, however, awarning signal should be sent to the operating centre as soon as thetemperature for rated operation is exceeded, since this indicates thatan overload situation exists and should be remedied.

FIG. 9 shows a further development of the control circuit in FIG. 7.Here a restricted control based on the temperature, aimed at maintainingthe terminal voltage at as acceptable a level as possible for as long aspossible by utilizing the thermal capacity of the stator to the maximum,is combined with a control of active and reactive power.

An output signal is thus generated in the unit ΔT“Prod” in the same wayas in the circuit according to FIG. 8. This signal is supplied to theamplifier and signal-processing unit 40, block 26 and gate 20 to achievethe same limiting control as in FIG. 8. The output signal from the unitΔT“Prod” is also supplied to the unit ΔP“Prod”, together with thevoltage U˜, whereupon a control signal ΔP order is obtained as outputsignal from the unit ΔP“Prod” in order to reduce the active power toU=ref, i.e. the terminal voltage equal to desired operating voltage oruntil the active power reaches a predetermined minimum power limit, asmentioned earlier. The reduction of active power is preferably commencedwhen either the core or the conductor temperature exceeds thetemperatures the machine is dimensioned for.

Yet another control possibility is based on starting a cooling machineto lower the iron and copper temperatures when either a current ortemperature limit is reached. This enables the machine to be loadedfurther.

What is claimed is:
 1. A synchronous machine having a rated power factorcomprising: a stator and a rotor, each having a corresponding thermallybased stator current limit and rotor current limit, and a stator windingand a rotor field winding, wherein said stator winding comprises aflexible high voltage cable having a selected cable radius, including aconductor formed of a plurality of conductor strands, an inner layerhaving semiconducting properties surrounding and contacting theconductor, a solid insulation surrounding and attached to the innerlayer and an outer layer having semiconducting properties surroundingand attached to the solid insulation, said cable having a flexibilitysufficient to achieve a bending radius of the cable of about 4 to about12 times the cable radius without causing detachment of the inner layer,the solid insulation and the outer layer, the rotor current limitintersecting the stator current limit at a power factor below the ratedpower factor, and the stator current limit being above the statorcurrent limit in a capability graph of the machine, and means responsiveto the stator temperature for limiting the stator current to avoidthermal damage to the stator winding and the rotor winding.
 2. Asynchronous machine according to claim 1, wherein the means for limitingcurrents comprises at least one of temperature-determining member todetermine the temperature of the stator located at a point critical toheating, and a current measuring device and a voltage measuring devicefor measuring stator current and voltage, and a control circuitresponsively connected thereto, to reduce the at least one of activepower and field current, if one of the temperature and stator currentand stator voltage exceeds predetermined limit values.
 3. A synchronousmachine as claimed in claim 2, wherein the temperature-determiningmembers comprise at least one measuring device arranged at a point inthe stator that is susceptible to heating, in order to measure thetemperature thereat.
 4. A synchronous machine as claimed in claim 3,wherein the measuring device is located on a slot wall inside a windingslot in the stator.
 5. A synchronous machine as claimed in claim 2,wherein the temperature-determining members comprise a temperatureestimator arranged to determine the temperature of the stator at acritical point for heating, in order to induce the control circuit toreduce the field current if the temperature determined exceeds apredetermined limit value.
 6. A synchronous machine as claimed in claim2, wherein the temperature-determining members comprise temperatureestimators arranged to determine the temperature in the cable.
 7. Asynchronous machine as claimed in claim 2, wherein the control circuitis responsive to commence reduction of the field current at a selectedtemperature below a maximum permissible stator temperature.
 8. Asynchronous machine as claimed in claim 2, wherein the control circuitis responsive to commence reduction of active power after thetemperature has been above a rated operating temperature below a maximumpermissible stator temperature, for a predetermined period of time.
 9. Asynchronous machine as claimed in claim 1, wherein the control circuitis responsive to control the field current if the stator currentexecutes the stator current limit so that the terminal voltage of themachine is equal to a desired operating voltage if the time during whichthe stator current has been above the stator current limit is shorterthan a maximum permissible time, and, if the maximum permissible timehas been exceeded, the control circuit is responsive to reduce the fieldcurrent until the stator current becomes equal to the stator currentlimit.
 10. A synchronous machine as claimed in claim 9, wherein thecontrol circuit is responsive to commence reduction of the field currentwith a selected time delay after the stator current limit has beenexceeded.
 11. A synchronous machine as claimed in claim 1, wherein thefield winding includes a number of extra turns in order to increase themagnetic pole voltage.
 12. A synchronous machine as claimed in claim 11,wherein a selected proportion of the extra turns are in the form ofcooling turns for the winding.
 13. A synchronous machine as claimed inclaim 1, wherein the field winding includes increased conducting area toproduce a relatively low current density in the winding.
 14. Asynchronous machine as claimed in claim 1, including cooling means forthe field winding.
 15. A synchronous machine as claimed in claim 1,comprising a cooling machine connectable therein to produce forcedcooling.
 16. A synchronous machine as claimed in claim 1, wherein thehigh-voltage cable has a diameter of about 20 mm to about 2000 mm and aconducting area of about 80 mm² to about 3000 mm².
 17. A synchronousmachine as claimed in claim 16, wherein said layers comprise materialshaving selected elasticity and selected coefficients of thermalexpansion such that changes in volume in the layers caused bytemperature fluctuations during operation are absorbed by the elasticityof the material so that the layers retain their adhesion to each other.18. A synchronous machine as claimed in claim 16, wherein the materialsin said layers have an E-modulus less than about 500 Mpa.
 19. Asynchronous machine as claimed in claim 16, wherein the coefficients ofthermal expansion for the materials in said layers are of substantiallythe same magnitude.
 20. A synchronous machine as claimed in claim 16,wherein the respective materials each have at least a selected strengthsuch that the adhesion between the layers is of at least the samemagnitude as the selected strength.
 21. A synchronous machine as claimedin claim 16, wherein each of the semiconducting layers comprises anequipotential surface.
 22. A synchronous machine as claimed in claim 1,wherein the rotor includes salient poles.
 23. A synchronous machine asclaimed in claim 1, wherein the rotor is cylindrical.
 24. A synchronousmachine as claimed in claim 16, wherein the materials in said layershave an E-modulus less than about 200 Mpa.
 25. A method for control of asynchronous machine according to claim 1, wherein the machine operateswith the stator current exceeding the thermally based stator currentlimit for a time period less than a selected maximum permissible timelimit, whereafter overload is reduced by reduction at least one ofactive power and the field current.
 26. A method according to claim 25,wherein the machine is capable of operating at overload with the statorcurrent exceeding the thermally based stator current limit by at least30% for at least 3 minutes without thermal damage, wherein the machinehas achieved rated temperature prior to the overload.
 27. A methodaccording to claim 25, wherein the machine is capable of operating atoverload with the stator current exceeding the thermally based statorcurrent limit by at least 30% for at least 5 minutes without thermaldamage, wherein the machine has achieved rated temperature prior to theoverload.
 28. A method according to claim 25, wherein the machine iscapable of operating at overload with the stator current exceeding thethermally based stator current limit by at least 50% for at least 5minutes without thermal damage, wherein the machine has achieved ratedtemperature prior to the overload.
 29. A method according to claim 25,wherein the machine is capable of operating at overload with the statorcurrent exceeding the thermally based stator current limit by at least80% for at least 15 minutes without thermal damage, wherein the machinehas achieved rated temperature prior to the overload.
 30. A methodaccording to claim 25, wherein the cable comprises a conductive core andan electric field confining insulating covering surrounding the core.31. A method for controlling a synchronous machine comprising a statorwith a stator winding and a rotor with a field winding, wherein thestator winding is wound of a high voltage cable formed with a selectedcable radius, including a conductor formed of a plurality of conductivestrands, an inner layer having semiconducting properties surrounding andcontacting the conductor, a solid insulation surrounding and attached tothe inner layer and an outer layer having semiconducting propertiessurrounding and attached to the solid insulation, forming said cablewith a flexibility sufficient to achieve a bending radius of the cableof about 4 to about 12 times the cable radius without causing detachmentof the inner layer, the solid insulation and the outer layer, and in therotor the field winding has thermally based rotor and stator currentlimits intersecting each other in a capability graph at a power factorvalue below a rated power factor value of the machine, and comprisingthe step of reducing active power if the stator current increasessufficiently to incur risk of thermal damage.
 32. A method as claimed inclaim 31, wherein when the stator current exceeds the stator currentlimit for a predetermined maximum permissible time, if the statorcurrent is above the stator current limit, reducing active power untilthe stator current becomes equal to the stator current limit, if thetime during which the stator current has been above the stator currentlimit is shorter than said maximum permissible time.
 33. A method asclaimed in claim 32, wherein if the stator current is above the statorcurrent limit for a time exceeding the maximum permissible time,reducing the active power and the field current until the stator currentis equal to the stator current limit.
 34. A method as claimed in 32, afurther including selecting the limit value for the power factor tozero.
 35. A method as claimed in claim 31, further including reducingthe active power in accordance with a ramp function.
 36. A method asclaimed in claim 35, further including selecting a derivative for theramp function to avoid power oscillations on the electric power networkand preventing damage to turbines and other parts of the electric powerproduction plant in which the synchronous machine is operated.
 37. Amethod as claimed in claim 35, further including selecting a derivativefor the ramp function, which is dependent on a time constant for warmingup the stator.
 38. A method as claimed in claim 35, further includingreducing the active power such that an acceptable terminal voltage ismaintained on the machine.
 39. A method as claimed in claim 31, furtherincluding reducing the active power in accordance with a ramp function,if the stator current has exceeded the stator current limit but is belowa predetermined second limit value above the stator current limit, andreducing the active power as fast as possible if the stator currentexceeds said second limit value.